1. Field of the Invention
The present invention relates generally to flexible circuits for semiconductor devices and, more specifically, to a flexible carrier substrate for use with semiconductor devices that facilitates high density stacking of the semiconductor devices.
2. State of the Art
Integrated circuit devices are commonly mounted on circuit boards and connected to conductive patterns formed on the circuit boards. Wire bonds may be used to interconnect the integrated circuit (IC) devices to traces of a conductive pattern formed on a circuit board. Each wire is bonded to both a bond pad of the IC device and a terminal pad formed at the end of a trace of the conductive pattern on the circuit board.
Wire bonding techniques are well known in the art and are highly reliable in most applications. Unfortunately, in a few applications, difficulties do occur. These applications exist when the IC device is small and the available area for connecting to the bond pads is limited. The process of reliably connecting wire bonds to small, closely-spaced bond pads is both tedious and expensive. Both ends of the wire bond must be accurately placed to avoid contacting adjacent pads on the IC device and the circuit board, respectively. Moreover, the wire must be sufficiently welded to the conductive bond pads to ensure a secure connection with good electrical contact and without damaging the IC device or the supporting circuit board.
To provide greater circuit density, circuit boards can be layered together in stacks and then interconnected electrically. This results in three-dimensional modules as the circuit boards are stacked one upon another. In stacking circuit boards one upon another, board production techniques become even more complex than before. This is because each layer may be different, requiring different circuit layouts and putting a strain on the ability of the assembly process to maintain dimensional tolerances that would not be as troublesome in a single layer interconnect layout assembly.
Another challenge in the art is an inability in some circumstances to provide a flat, smooth surface on which to mount a printed circuit board. Accordingly, flexible circuit boards have been developed to promote both lighter structures and greater adaptability to conforming to nonuniform surfaces. Unfortunately, the arrival of flexible circuit boards has resulted in other problems, such as the problem in joining several boards while effecting and maintaining a proper interconnection between the respective boards. Further, in some applications, where preclusion of ICs mounted on a lower circuit board from touching a higher circuit board is required, providing a rigid assembly to support the stacked circuit boards is useful. Unfortunately, this approach compromises the flexibility that would otherwise allow the circuit boards to conform to a non-planar surface.
One example of integrated circuit devices mounted upon flexible, stackable circuit boards to form semiconductor modules is disclosed in U.S. Pat. No. 5,440,171 entitled xe2x80x9cSemiconductor Device with Reinforcement,xe2x80x9d issued Aug. 8, 1995. The ""171 patent discloses a basic structural unit that uses a flexible circuit board made from a polyimide film with circuit lines formed on both sides, typically using copper foil. A supporting frame is provided and is bonded to the flexible circuit board with a heat resistant resin, such as a polyimide resin. Electrical connection is possible between the flexible circuit board and the support frame, which may include a plurality of layers. Conductive through holes are provided so that electrical continuity may be maintained between a semiconductor device mounted upon the flexible circuit board and either at least one other semiconductor device mounted on another flexible circuit board stacked within the module assembly, or an external structure upon which the entire basic structural unit is mounted. The semiconductor devices are electrically connected to electrodes formed on the support frame.
Although a semiconductor device in the ""171 patent is mounted on a flexible circuit board that is stackable with other circuit boards in a three-dimensional arrangement, the support frame attaching the stackable circuit boards one to another is made from a rigid material that does not allow for any bending at all. For example, one type of frame material suggested in the ""171 patent is a ceramic such as alumina or silicon nitride. Such materials may be used for high thermal conductivity to promote heat transfer from high power consumption semiconductor devices mounted on the flexible circuit board. However, because the support frame is made from an extremely rigid and non-flexible material, the entire semiconductor structure utilizing the stackable circuit boards and support frames must necessarily comprise a series of parallel, superimposed layers and must be mounted upon a substantially planar surface. This prevents the assembly from conforming to non-planar surfaces.
Accordingly, what is needed is a flexible circuit board having an associated support frame that overcomes the problems of the conventional practice of utilizing a rigid support frame and is readily adaptable for stacking in multiple layers. Additionally, the improved flexible circuit board with stackable support frame should be more easily assembled and mounted than was possible with prior art structures when disposed upon non-planar surfaces.
According to the present invention, a flexible substrate module or assembly is disclosed that facilitates stacking of multiple flexible carrier substrates to simplify the assembly of high density electronic systems. Integrated circuit semiconductor devices in the form of chips or dice are connected active surface side down to a flexible carrier substrate in a so-called xe2x80x9cflip-chipxe2x80x9d orientation using solder bumps or other discrete conductive bumps or elements. Such conductive connecting elements may be formed either on the die itself or on the flexible substrate. After the dice are placed on and secured to the flexible carrier substrate, a frame, preferably offering a significant degree of flexibility, is applied to the flexible carrier substrate to surround the perimeter thereof. The flexible frame includes conductive paths therethrough in the form of conductively-plated through holes, electrical conductor-filled vias, or preformed conductive elements, which conductive paths connect to circuit traces on the flexible carrier substrate extending from the electrical connections of the dice thereto on the interior region of the flexible carrier substrate to the flexible carrier substrate perimeter. This feature permits operational stacking of multiple flexible carrier substrates for cooperation between semiconductor dice mounted on different-level flexible carrier substrates and between all components of the stacked assembly and external, higher-level packaging by providing electrical interconnection between the various flexible carrier substrates. Since the flexible carrier substrates may be extremely flexible, they may be formed to a radius of substantially any given curvature, providing the ability to conform to various non-planar, arcuate or non-arcuate, regular or irregular surfaces. Further, the flexible carrier substrates exhibit substantial flexibility so as to provide significant bending angles, permitting mounting of the flexible carrier substrates to many structures having non-planar surfaces. Furthermore, since the perimeter frame as well as the carrier substrate may be flexible, multiple flexible carrier substrate modules, each comprising a flexible carrier substrate and associated frame, may be stacked one on top of another in superimposed arcuate configurations, wherein the top module may have a smaller or larger radius of curvature than the bottom module and any modules in between have progressively differing radii from bottom to top position.
Mounting multiple modules in a stacked configuration with differing module radii may be accomplished by attaching first ends of a plurality of modules comprising superimposed flexible carrier substrates and support frames and then sequentially attaching second ends of the modules after a given radius is established for each lower module. In an alternative assembly technique, the first and second ends of a first flexible carrier substrate may be attached to a desired surface. Next, a flexible support frame is then attached to a first end and then a second end of the first flexible carrier substrate. Next, a second flexible carrier substrate may be attached to the first and second ends of the first frame and the process repeated in layers until a desired, completed structure is formed.